Where the Glacier Stopped: Des Moines Ground and Its Flood Record

The Des Moines Lobe, Iowa's last glacial advance, halted at what is now Des Moines 12,000 to 14,000 years ago and left mostly glacial till (Iowa Geological Survey). Till is the parent material for about 60% of Polk County's soil acreage, and a shallow saturated zone is the most common reason those soils are rated limited for basements (USDA NRCS).

What glacial till means under a house

Till is what an ice sheet leaves when it melts in place: a compact, unsorted mix of clay, silt, sand and pebbles. The Iowa Geological Survey describes the Des Moines Lobe as a low-relief plain of till, marked by moraines at the limits of its advances, whose surface drainage never fully developed; lakes, marshes and prairie potholes were the result. The ice also set the present course of the Raccoon River.

For a basement, dense till drains slowly. Water reaching the soil beside a wall tends to stay there, and close to half of Polk County's major soil acreage is classed as poorly or somewhat poorly drained (USDA NRCS). Clarion, Webster and Nicollet are the county's most extensive soils.

How the soil survey rates Polk County for basements

The USDA Natural Resources Conservation Service rates every mapped soil for "dwellings with basements." Across Polk County's rated acreage:

• Very limited: about 56%
• Somewhat limited: about 41%
• Not limited: about 3%

The reasons, from most to least common, are depth to a saturated zone, shrink-swell, flooding and slope. About a tenth of the county, much of it mapped as urban land where the original soil was disturbed, is not rated at all.

Two rivers, two record floods

The Raccoon River at Fleur Drive has a flood stage of 12 feet; it crested at a record 26.8 feet on July 11, 1993, and at 24.66 feet on June 13, 2008. The Des Moines River at SE 6th Street, flood stage 24 feet, reached 34.29 feet on July 11, 1993 and set its record of 35.55 feet on June 13, 2008 (NWS river gauges). In 1993 Raccoon River water overran the protection around the Des Moines Water Works and shut the plant, cutting off water to about a quarter-million people for nearly two weeks (NWS). Polk County was included in federal disaster declarations both years (FEMA DR-996 and DR-1763).

Smaller streams matter too: Walnut Creek at 63rd Street hit its record of 19.08 feet on June 30, 2018, and Fourmile Creek at Easton Boulevard its record of 17.51 feet on July 1, 2018 (NWS).

What it means for your basement

On high ground, river crests matter less than groundwater, roof water and soil that holds moisture against the wall. On low ground near the rivers and creeks, check the flood map before any big basement project. You can look up the soil under your own lot in the USDA's Web Soil Survey. Then read about waterproofing a Des Moines basement and repairing cracked or bowing walls, and compare prices in the foundation repair cost guide.